1. Field of the Invention
The present invention relates to floating roofs for use in storage tanks for oil or other liquid petrochemical products.
2. History of the Prior Art
Storage tanks for oil and other liquid petrochemical products are typically provided with a floating roof. The floating roof floats on the liquid product and seals the tank to prevent the escape of harmful vapors when provided with one or more seals extending between the outer periphery of the floating roof and the inner wall of the tank.
Such floating roofs are usually circular in shape and of relatively thin, generally planar configuration. The roofs may assume a number of different basic forms. One particular type of floating roof, which is popular because it lends itself to lightweight construction using aluminum and other relatively lightweight materials, consists of a frame or deck supported by pontoons mounted at the underside thereof. The frame is typically comprised of an outer circular frame member and a plurality of beams or girders disposed therein and attached thereto. The girders, which are typically of aluminum construction, are arranged into a grid pattern of spaced-apart, parallel main girders and cross girders to provide the floating roof with adequate strength and rigidity while at the same time minimizing the weight of the roof. The pontoons are coupled to support legs mounted at the underside of the floating roof. Deck sheeting in the form of sheets of aluminum or other lightweight material is secured over the girders to provide the floating roof with a top surface.
An example of a prior art floating roof of the type described is provided by U.S. Pat. No. 4,244,487 of Kern, which patent issued Jan. 13, 1981 and is entitled "Floating Cover Having Pivotally Connected Flotation Pontoons". In the floating roof described in the Kern patent, a plurality of support legs are mounted at the underside of a sheet metal decking and are coupled to flanges extending from the opposite ends of a plurality of pontoons. Other support legs are mounted by coupling to pivotal interconnections of flanges extending from the ends of adjacent pontoons. The pontoons are coupled to the sheet metal decking by one of a pair of opposite beams, which sandwich overlapping deck sheets therebetween using bolts as fasteners.
A major problem with floating roofs of the type described in the Kern patent relates to the fact that the pontoons are joined together, end-to-end, as well as to the support legs. This results in considerable stress being placed on the pontoons, which must function as main structural beams. Because of the design of the floating roof deck, such deck is incapable of providing much support for the pontoons or for the support legs. Moreover, the pontoons must provide primary support for the support legs, which are only minimally supported by the deck in cases where they are attached to the deck or not at all in other cases. To prevent rupture of the flanges welded to caps at the ends of the pontoons, particularly under conditions of cyclic loading such as may be due to turbulence in the liquid product, the pontoons are pivotally coupled to the opposite end flanges. Because the overlapping sheet metal decking is clamped by bolts extending through the opposite beams, such bolts penetrate the vapor space above the liquid product in the tank and pose a leakage problem if the bolts are not sealed or become loose.
A further problem in the construction of those prior art floating roofs which employ a grid of girders in the deck thereof relates to the substantial nature of the grid network formed by the girders. Because such roofs typically have a diameter of 50feet or more, the deck frame thereof may require assembly of many main girders and cross girders. Attachment of such girders requires that many holes be drilled so that the girders can be bolted or otherwise joined together. This is typically a very time consuming process. Moreover, once girders are joined at a particular location, they cannot be adjusted or relocated without drilling more holes. The deck sheeting is typically mounted on the frame or deck by bolting to the girders, again requiring that more holes be drilled. Because such holes extend through the girders to the underside of the floating roof, they pose a problem of leakage, even after bolts are secured therein to mount the deck sheeting thereon. Such leakage not only causes air pollution but also results in loss of the liquid product which can represent a financial loss as well.
Floating roofs of the type described must be provided with a plurality of support legs at the underside thereof. Such support legs contact the bottom of the tank to hold the floating roof above the bottom when the tank is empty or nearly empty of liquid product. This prevents the floating roof itself from resting on the tank bottom, with possible damage to the pontoons or other portions of the floating roof. The legs are typically adjustable in order to compensate for non-level tank bottoms. The legs are adjusted during a levelling process before the tank is filled with liquid product. Typically, the leg assemblies are coupled to the ends of the pontoons and derive their major support therefrom, as in the case of the Kern patent described above. Although the leg assemblies may also be coupled to the deck sheeting, most of the support typically comes from the flanges which extend from end caps at the opposite ends of the pontoons to support the leg assemblies. This results in considerable stress on the flanges, the end caps and the pontoons themselves.
Floating roofs of the type described are desirably provided with means for venting the underside of the roof when it is adjacent the bottom of the tank. Normally, the floating roof must maintain an airtight seal, to prevent the escape of harmful vapors. However, venting of the floating roof when it is resting on the tank bottom allows air which may accumulate at the underside of the floating roof to escape, while at the same time breaking or venting a vacuum which may occur at the underside of the roof. The air build up and vacuum may occur when filling of the tank with liquid product is begun, or when the liquid product is being drained from the tank.
In floating roofs of the type described, the pontoons may be coupled to the underside of the frame by arrangements which employ an elongated metal strap in conjunction with a two-piece saddle and a saddle bar. The saddle bar, which is placed inside the girder, is required in order to determine the positions of opposite portions of the two-piece saddle which receive and support the pontoon. Opposite ends of the strap are bolted through the opposite portions of the two-piece saddle to the opposite ends of the saddle bar within the girder, to secure the metal strap and the two-piece saddle in place at the underside of the girder.
In floating roofs of the type described, the opposite ends of each pontoon are typically provided with end caps which are welded in place thereon to secure the end caps to the hollow tubes forming the pontoons. The welding must be done carefully in order to make the pontoons airtight and liquidtight and at the same time make the pontoons capable of supporting the legs via flanges attached to the end caps. Because the end caps are placed over the open ends of the pontoons for welding, the welding process is made difficult. Moreover, the strength requirements imposed on the pontoons and on the end caps thereof, due to the coupling of the legs thereto, are substantial.
Accordingly, it would be desirable to provide a floating roof having pontoons which are not joined together and which are not required to support legs or other appendages to the roof. Without the pontoons having to function as main structural beams, the pontoons need not be joined together, and rigid joint support legs are supported entirely by the deck frame and which, in any event, do not require the pontoons for their support. Among other things, this would eliminate the problem of fatigue at the very critical connection point of the leg to the pontoon, with resulting leakage in the pontoon. It would still furthermore be desirable to provide a floating roof in which the sheet metal decking is mounted to the deck frame by an arrangement which does not penetrate the vapor space below and therefore poses no danger of leakage, such as may be caused by loosening of the connecting bolts or the corrosion of such bolts when exposed to the vapors.
Accordingly, it would be desirable to provide a floating roof in which girders within the frame or deck can be easily coupled to one another at any of a variety of locations along the lengths thereof and in which the deck sheeting can be mounted thereon, without the need for drilling holes. Such floating roof should be capable of coupling the support legs to the girders, so that the legs do not have to support the pontoons. Moreover, the design of the floating roof should provide considerable flexibility in terms of where the legs can be mounted. The design of the floating roof should also allow manway assemblies, bleeder vent assemblies, and other required roof penetrations, to be easily and simply mounted in any one of a variety of different locations throughout the floating roof. The pontoons should be capable of mounting at the underside of the girders using a simplified mounting arrangement. The end caps of the pontoons should desirably be designed to facilitate the process of welding them to the opposite ends of the pontoons to achieve the watertight seal, while at the same time enhancing the structural integrity of the hollow cylindrical configuration of the pontoons.